1. Field
The current disclosure relates to digital power amplifiers, and more specifically, but not exclusively, to digital power amplifiers for radio-frequency transmission systems.
2. Description of the Related Art
Radio-frequency (RF) transmitters communicate with RF receivers using radio-frequency electromagnetic signals. A typical RF transmitter includes a processing module and an antenna. A digital processing module uses relatively low-power digital signals. The signals transmitted by the antenna, however, need to be relatively high-powered and analog in order to be picked up by antennas in the RF receivers. Consequently, a power amplifier is typically used by RF transmitters on outgoing signals between the processing module and the antenna.
Conventional power amplifiers that use analog components provide relatively low power efficiencies. Digital power amplifiers, such as so-called class-D and class-S amplifiers, can be significantly more efficient than analog power amplifiers. Increasing the power-use efficiency may reduce initial costs, operating costs, and maintenance costs of an RF transmitter. The design and operation of various analog and digital power amplifiers are described in Stephen Ralph's “Class-S Power Amplifier For Use In Mobile Phone Basestations,” National University of Ireland Maynooth, 2007, incorporated herein by reference in its entirety.
FIG. 1 shows a simplified block diagram of conventional power amplification system 100 of an RF transmitter with a digital processing module. System 100 comprises digital signal source 101 which provides multi-bit, relatively low-frequency, digital baseband (BB) signal 101a to bit-stream generator 102. Bit-stream generator 102 may be, for example, a pulse-width modulator or a sigma-delta modulator. Bit-stream generator 102 generates a digital, low resolution (e.g., 1-bit), radio-frequency switching signal, or bit stream, 102a, which is derived from digital signal 101a based on a transfer function implemented by bit-stream generator 102.
Switching signal 102a is at a higher frequency but a lower resolution than digital signal 101a, because the transformation of signal 101a into signal 102a introduces quantization noise into signal 102a. The transfer function of bit-stream generator 102 may be used for spectral shaping and may be set so that the location, on a frequency scale, of the quantization noise is moved away from a frequency band of interest to an outlying frequency band. Bit-stream generator 102 performs frequency up-conversion from a baseband frequency to a radio frequency and transforms high-resolution digital signal 101a into low-resolution digital signal 102a while maintaining a high signal-to-noise ratio (SNR) in the frequency band of interest.
Switching power amplifier 103 acts as a high-fidelity RF digital-to-analog converter that outputs amplified RF signal 103a based on signal 102a. Signal 103a is then provided to bandpass filter 104, which substantially passes through the frequency band of interest while substantially blocking other frequencies, thereby substantially filtering out the quantization error introduced by bit-stream generator 102. Bandpass filter 104 outputs analog signal 104a, which is provided to antenna 105 for transmission. Note that system 100 uses oversampling, where the sampling frequency of bit-stream signal 102a is several times higher than the desired bandwidth of the analog RF output signal 104a. 
FIG. 2 shows a circuit diagram of one conventional implementation of switching power amplifier 103 of FIG. 1, which functions to provide output signal 103a, an amplified analog version of its digital input signal 102a. Amplifier 103 comprises n-channel field-effect transistor (FET) 201 and p-channel FET 202, both of whose gates are controlled by input signal 102a. FETs 201 and 202 may be power transistors using, for example, GaN, LDMOS, or GaAs technologies. The drain terminal of FET 201 is connected to voltage source 203. The drain terminal of FET 202 is connected to common (i.e., ground) voltage 204. The source terminals of FETs 201 and 202 are connected together to generate output signal 103a. Protection diode 205 is connected between the drain and source of FET 201, while protection diode 206 is connected between the drain and source of FET 202. Output signal 103a may go through a broad-band RF band-pass filter (not shown) before being output by amplifier 103. A broad-band RF band-pass filter may be implemented as an RF tank circuit, also known as an LC circuit.
FIG. 3 shows a simplified block diagram of one implementation of bit-stream generator 102 of FIG. 1. Bit-stream generator 102 of FIG. 3 is implemented as a sigma-delta modulator and comprises upsampler 301, loop filter 302, and quantizer 303. A sigma-delta modulator, as its name suggests, performs a summation of differences. Upsampler 301 receives input digital signal 101a at sampling frequency Fs, increases the sampling frequency by a factor P, and outputs digital signal 301a at sampling frequency P*Fs. Signal 301a is one of the inputs to loop filter 302 with the other being signal 102a, the 1-bit output of bit-stream generator 102. Loop filter 302 comprises a comparator and an integrator (not shown) and performs a summing of differences between input 301a and input 102a and outputs the result as signal 302a to quantizer 303. Quantizer 303 outputs a high or low value—corresponding to a positive or negative value, respectively—depending on whether 302a is above or below a set threshold. 1-bit output signal 102a has a sampling frequency of P*Fs.